Method of Manufacture for a Lightweight, High-Precision Silicon Carbide Mirror Assembly

ABSTRACT

An aerospace mirror having a reaction bonded (RB) silicon carbide (SiC) mirror substrate, and a SiC cladding on the RB SiC mirror substrate forming an optical surface on a front side of the aerospace mirror. A method for manufacturing an aerospace mirror comprising obtaining a green mirror preform comprising porous carbon, silicon carbide (SiC), or both, the green mirror preform defining a front side of the aerospace mirror and a back side of the aerospace mirror opposite the front side; removing material from the green mirror preform to form support ribs on the back side; infiltrating the green mirror preform with silicon to create a reaction bonded (RB) SiC mirror substrate from the green mirror preform; forming a mounting interface surface on the back side of the aerospace mirror from the RB SiC mirror substrate, and forming a reflector surface of the RB SiC mirror substrate on the front side of the aerospace mirror. Additionally, the method can comprise cladding the reflector surface of the RB SiC mirror substrate with SiC to form an optical surface of the aerospace mirror.

RELATED APPLICATIONS

This is a divisional application of U.S. application Ser. No.15/993,513, filed May 30, 2018, entitled “Method of Manufacture for aLightweight, High-Precision Silicon Carbide Mirror Assembly”, which isincorporated by reference in its entirety herein.

BACKGROUND

Space-based imaging systems (e.g., in telescopes commonly found onsatellites) are often configured as reflector-type imaging systems. Suchimaging systems require large primary mirrors (i.e., mirror diametergreater than ˜0.5 meter) with large apertures to achieve a desiredresolution and/or signal-to-noise ratio (SNR). Spaced-based imagingsystems are susceptible to environmental (e.g., temperature) variationsthat can reduce image quality. Mirrors used in such imaging systems aretherefore designed to reduce sensitivity to environmental variation. Inaddition, surface distortion of a mirror's optical surface, or “figureerror,” is typically held to very tight tolerances (e.g., a fraction ofa millionth of an inch). The larger the mirror the more difficult it isto avoid distortion and meet the tolerance requirements. Materials anddesigns for high-precision, space-based optical applications arecharacterized by a low coefficient of thermal expansion (CTE), lightweight, and high stiffness in order to meet design requirements. Onetypical mirror design utilizes near-zero CTE material for optics (e.g.,ultra-low expansion (ULE) glass mirrors) in the system. Silicon carbide(SiC) is also an emerging material for use in high-precision optics.

BRIEF DESCRIPTION OF THE DRAWINGS

Features and advantages of the invention will be apparent from thedetailed description which follows, taken in conjunction with theaccompanying drawings, which together illustrate, by way of example,features of the invention; and, wherein:

FIG. 1A illustrates a front view of an aerospace mirror in accordancewith an example of the present disclosure.

FIG. 1B illustrates a back view of the aerospace mirror of FIG. 1A, inaccordance with an example of the present disclosure.

FIG. 2 is a detailed cross-sectional view of the aerospace mirror ofFIG. 1A taken along line A-A, in accordance with an example of thepresent disclosure.

FIG. 3 is a detailed cross-sectional view showing a mounting interfacesurface of the aerospace mirror of FIG. 1A taken along line B-B, inaccordance with an example of the present disclosure.

FIG. 4 illustrates a method for manufacturing an aerospace mirror inaccordance with an example of the present disclosure.

FIG. 5 illustrates a method for preparing an optical surface of anaerospace mirror in accordance with an example of the presentdisclosure.

FIG. 6 illustrates a front view of an aerospace mirror manufacturingassembly in accordance with an example of the present disclosure.

FIG. 7 illustrates a side view of the aerospace mirror manufacturingassembly of FIG. 6, in accordance with an example of the presentdisclosure.

FIG. 8 illustrates a back view of the aerospace mirror manufacturingassembly of FIG. 6, in accordance with an example of the presentdisclosure.

FIG. 9 is a detailed view of the aerospace mirror manufacturing assemblyof FIG. 6 taken along line C-C, in accordance with an example of thepresent disclosure.

Reference will now be made to the exemplary embodiments illustrated, andspecific language will be used herein to describe the same. It willnevertheless be understood that no limitation of the scope of theinvention is thereby intended.

DETAILED DESCRIPTION

As used herein, the term “substantially” refers to the complete ornearly complete extent or degree of an action, characteristic, property,state, structure, item, or result. For example, an object that is“substantially” enclosed would mean that the object is either completelyenclosed or nearly completely enclosed. The exact allowable degree ofdeviation from absolute completeness may in some cases depend on thespecific context. However, generally speaking the nearness of completionwill be so as to have the same overall result as if absolute and totalcompletion were obtained. The use of “substantially” is equallyapplicable when used in a negative connotation to refer to the completeor near complete lack of an action, characteristic, property, state,structure, item, or result.

As used herein, “adjacent” refers to the proximity of two structures orelements. Particularly, elements that are identified as being “adjacent”may be either abutting or connected. Such elements may also be near orclose to each other without necessarily contacting each other. The exactdegree of proximity may in some cases depend on the specific context.

An initial overview of the inventive concepts are provided below andthen specific examples are described in further detail later. Thisinitial summary is intended to aid readers in understanding the examplesmore quickly, but is not intended to identify key features or essentialfeatures of the examples, nor is it intended to limit the scope of theclaimed subject matter.

Although large ULE glass mirrors have been effectively utilized inhigh-precision, space-based optical applications, these mirrors areheavy and difficult to produce. The low stiffness-to-weight ratio of ULEglass material results in self-weight deflection in large ULE glassmirrors (e.g., primary mirrors in reflector telescope systems), whichcomplicates alignment and testing on the ground. SiC, on the other handhas excellent mechanical properties, but cannot be polished to thenecessary surface finish. A SiC mirror therefore requires a claddingmaterial, such as amorphous elemental silicon, to provide an acceptablesurface finish of the final mirror. In addition, some forms of SiC, suchas sintered SiC commonly utilized in SiC mirrors, exhibit shrinkage(e.g., greater than 35%) in the manufacturing process, thus requiringover-processing and limiting the size of the mirror that can beproduced. The materials currently used to meet the requirements ofhigh-precision, space-based optical systems are expensive to manufactureand require long lead times to produce. A trade-off between weight,thermal expansion, and stiffness is often required to meet cost andschedule constraints.

Accordingly, an aerospace mirror and associated manufacturing methodsare disclosed that provide a light-weight, high-stiffness, and lowthermal expansion mirror with excellent optical properties, which can bemanufactured at lower cost and on a shorter schedule than with legacymaterials. The aerospace mirror can include a reaction bonded (RB) SiCmirror substrate, and a SiC cladding on the RB SiC mirror substrateforming an optical surface on the front side of the aerospace mirror.

A method for manufacturing an aerospace mirror can comprise obtaining agreen mirror preform comprising porous carbon, SiC, or both, the greenmirror preform defining a front side of the aerospace mirror and a backside of the aerospace mirror opposite the front side. The method canalso comprise removing material from the green mirror preform to formsupport ribs on the back side. The method can further compriseinfiltrating the green mirror preform with silicon to create a RB SiCmirror substrate from the green mirror preform. The method can stillfurther comprise forming a mounting interface surface on the back sideof the aerospace mirror from the RB SiC mirror substrate, and forming areflector surface of the RB SiC mirror substrate on the front side ofthe aerospace mirror. Additionally, the method can comprise cladding thereflector surface of the RB SiC mirror substrate with SiC to form anoptical surface of the aerospace mirror.

A method for preparing an optical surface of an aerospace mirror is alsodisclosed. The method can comprise obtaining an aerospace mirror havingan optical surface on a front side and a mounting interface surface on aback side, the mounting interface surface being operable to mount orfacilitating the mounting of the aerospace mirror to an external supportstructure in a final installation by coupling with a final installationmounting structure. The method can also comprise assembling theaerospace mirror to a test support base by coupling a test mountingstructure to the mounting interface surface, wherein the test mountingstructure is the same as, or equivalent to, or otherwise corresponds to,the final installation mounting structure. The method can furthercomprise measuring the optical surface (i.e., while attached to the testmounting structure). Additionally, the method can comprise machining theoptical surface.

In addition, an aerospace mirror manufacturing assembly is disclosed.The assembly can comprise a test support base. The assembly can alsocomprise a test mounting structure. Additionally, the assembly cancomprise an aerospace mirror mounted to the test support base via thetest mounting structure. The aerospace mirror can have an opticalsurface on a front side and a mounting interface surface on a back sidecoupled to the test mounting structure. The mounting interface surfacecan be operable to mount the aerospace mirror to an external supportstructure in a final installation by coupling with a final installationmounting structure. The test mounting structure can be the same as, orequivalent to, or otherwise correspond to, the final installationmounting structure.

To further describe the present technology, examples are now providedwith reference to the figures. With reference to FIGS. 1A and 1B, oneembodiment of an aerospace mirror 100 is schematically illustrated infront and back views showing respective front (FIG. 1A) and back (FIG.1B) sides of the mirror 100, which are opposite one another. Detailedcross-sectional views of the mirror 100 are shown in FIGS. 2 and 3. Insome embodiments, the mirror 100 can be configured as a primary mirrorin a reflective mirror system, which may be used in optical telescopes(e.g., high resolution imaging systems) and infrared sensors.

In general, the mirror 100 can include a mirror substrate and areflective or optical surface on the front side of the mirror 100disposed on or otherwise supported by the substrate. For example, asillustrated in cross-section in FIG. 2, a mirror substrate 110 can havea reflector surface 111 on a front side 101 a of the mirror 100. Anoptical surface 112 can be formed on the reflector surface 111 of thesubstrate 110. The reflector surface 111 can provide the basic structurefor the mirror's optical surface 112, which can be formed of a thinlayer of material or cladding 113 disposed on the reflector surface 111,as discussed in more detail below. In some embodiments, the reflectorsurface 111 can form one side of a facesheet 114 of the mirror substrate110.

The mirror substrate 110 can have any suitable configuration. In oneaspect, the mirror substrate 110 can be configured to includeweight-reduction structural features, such as support ribs 115 on theback side 101 b of the mirror 100 (e.g., an “open back” configuration),which can provide support for the reflector surface 111 and opticalsurface 112 of the mirror 100. For example, as shown in FIG. 2, the ribs115 can extend from the facesheet 114 to provide a structural backingfor the reflector surface 111 and the mirror's optical or reflectivesurface 112. The ribs 115 can be in any suitable arrangement orconfiguration to provide adequate structural support for the mirror 100.As illustrated in FIG. 1B, the ribs 115 can be arranged in a triangularisogrid pattern, which can provide a lightweight and stiff substratestructure for the mirror's optical or reflective surface 112. However,other rib patterns may be utilized, and are contemplated herein.

The mirror 100 can also include one or more mounting interface surfaces(e.g., bond pads) on the back side 101 b of the mirror 100 to mount themirror 100 to an external support structure for the mirror 100, such asa satellite (e.g., via a mounting structure, such as a strut (notshown)). In one aspect, mounting interface surfaces can be formed in oron the mirror substrate 110. In the embodiment shown in FIG. 3, amounting interface surface 116 (e.g., a bond pad) can be formed on a rib115 of the substrate 110, for example, to facilitate bonding with anattachment fitting. Examples of such mounting interface surface and ribconfigurations are disclosed in United States Patent ApplicationPublication No. 2017/0055731, which is incorporated herein by referencein its entirety. The rib 115 can have any suitable profile 117 a-c,which may be configured to facilitate coupling with an attachmentfitting (i.e., a mounting structure) and/or to reduce deformation due tothe coupling with the attachment fitting. In other embodiments, amounting interface surface 116 can define a cylindrical hole (not shown)at an intersection of the support ribs 115 on the back side 101 b of themirror. The cylindrical hole can be used to bond or screw a metal insertor fitting to the mirror 100 for attachment to an external supportstructure.

In one aspect, the mirror substrate 110 can comprise a reaction bonded(RB) silicon carbide (SiC) material. In another aspect, the opticalsurface 112 is formed from a SiC cladding 113 on the RB SiC mirrorsubstrate 110. In particular, the SiC cladding 113 can comprise achemical vapor deposition (CVD) SiC material. In some embodiments, themirror 100 can include an optical coating 118 (e.g., ahigh-reflectivity, thin film coating) on the optical surface 112, asshown in FIG. 2.

A method 202 is outlined in FIG. 4 for manufacturing an aerospacemirror, such as the mirror 100 constructed of an RB SiC substrate 110and a SiC cladding 113 for the optical surface 112. In general, themethod comprises obtaining a green mirror preform 220, removing materialfrom the green mirror preform 221, infiltrating the green mirror preformwith silicon to create an RB SiC mirror substrate from the green mirrorpreform 222, selectively removing material to form precision features inthe RB SiC mirror substrate 223, and cladding the RB SiC mirrorsubstrate with SiC to form an optical surface 224.

The green mirror preform obtained at step 220 can comprise any materialor combination of materials (e.g., porous carbon, SiC, plasticizers,fillers, etc.)

suitable for reaction bonding by infiltration with silicon to form RBSiC material (e.g., comprising a SiC fraction of 70% and a Si filling of30%, although other relative compositions are contemplated). The greenmirror preform can be fabricated utilizing any suitable technique orprocess known in the art (e.g., made of a dried SiC and carbon slurry).RB SiC is a high specific stiffness, low thermal expansion material withexcellent mechanical properties for an aerospace mirror. The greenmirror preform can have any suitable shape or configuration and can bemade or constructed by any suitable process known in the art. Typically,the green mirror preform will have a shape that corresponds to an outerboundary shape of the final mirror substrate. Thus, the green mirrorpreform can define the general shape of the mirror as well as the frontand back sides of the mirror. The green mirror preform may bemanufactured to rough tolerances and sized to exceed the final size ofthe mirror substrate sufficient to allow for material removal in thegreen state, shrinkage due to silicon infiltration and formation of RBSiC, and selective material removal in the “hardened” RB SiC state toform precision features. One advantage of RB SiC material is lowshrinkage from the green state when compared to other types of SiC, suchas sintered SiC.

Material can be removed from the green mirror preform at step 221 toform various features of the mirror substrate 110. For example, materialcan be removed from the green mirror preform to form support ribs 115 onthe back side 101 b of the mirror 100 (e.g., in an isogrid or otherpattern) and to form a back side of the facesheet 114, which can reduceweight and areal density of the mirror 100. In some embodiments, thematerial can be removed from the front side 101 a of the green mirrorpreform to form a reflector surface 111 of the mirror substrate 110. Inother embodiments, the green mirror preform can be provided with asurface corresponding to the reflector surface 111 of the substrate 110that can be formed into RB SiC without prior material removal in thegreen state. Material can be removed from the green mirror preformutilizing any suitable technique or process. One benefit of the greenstate material for RB SiC is the speed and ease in which it can bemachined. Thus, typically, material will be removed from the greenmirror preform by machining (e.g., milling). In some embodiments, highspeed machining can be utilized. The ability to rapidly machine thematerial in the green state can significantly reduce the cost andschedule of producing a large, lightweight mirror blank from SiC byother methods, such as by casting to final dimensions.

Once material has been removed from the green mirror preform at step221, the green mirror preform can be infiltrated 222 with silicon tocreate an RB SiC mirror substrate 110 from the green mirror preform.Reaction bonded silicon carbide is made by a chemical reaction betweencarbon or graphite with gaseous and/or molten silicon. Siliconinfiltration to form reaction bonded SiC can be accomplished by anysuitable technique or process known in the art, such as infiltrationwith silicon in a gaseous and/or molten state. The silicon reacts withthe carbon to form silicon carbide (additional SiC if some was presentin the preform). The reaction product bonds the silicon carbideparticles. Any excess silicon fills the remaining pores in the body andproduces a dense SiC—Si composite. Due to the left-over traces ofsilicon, reaction bonded silicon carbide is often referred to assiliconized silicon carbide.

With a mirror substrate 110 made of RB SiC, material can be selectivelyremoved to form at step 223 small precision features and/or criticalfeatures in the mirror substrate 110 that meet dimensional, form, and/orsurface finish requirements. For example, one or more mounting interfacesurfaces 116 (e.g., small bond pads) can be formed on the back side 101b of the mirror 100 from the RB SiC mirror substrate 110. In addition,the reflector surface 111 of the RB SiC mirror substrate 110 can beformed on the front side 101 a of the mirror 100, which may be aprecision and/or critical feature due to its impact on optical imagequality. Material can be removed from the RB SiC mirror substrate 110utilizing any suitable technique or process, such as machining (e.g.,milling, lapping, grinding, etc.). Because RB SiC is a hard materialthat is difficult and time-consuming to machine, machining of thehardened SiC can be limited to only high-precision features (e.g.,mounting interfaces and optical-related surfaces) that may not beaccurately formed following green state machining and subsequentdeformation (e.g., shrinkage, although minimal) due to reaction bondingof the silicon carbide. The amount of material removed during such finemachining of the hard SiC material can be minimized by the design of thegreen mirror preform and/or the location or amount of material removedfrom the green mirror preform prior to forming RB SiC. The strategicremoval of material in the green state and in the hardened SiC state canprovide a lightweight mirror with a low areal density while minimizinghigh-precision machining time on hard SiC.

Once the reflector surface of the RB SiC mirror substrate is in anacceptable condition, the mirror substrate can be clad at step 224 withSiC to form the optical surface 112 of the mirror 100. In someembodiments, the SiC cladding 113 can be applied or formed by a chemicalvapor deposition (CVD) process (e.g., about 0.005 inches thick), aprocess in which volatile compounds containing carbon and silicon arereacted at high temperatures in the presence of hydrogen. Cladding theRB SiC mirror substrate 110 with CVD SiC can facilitate forming theoptical surface 112 because CVD SiC material is easier to polish than RBSiC material. CVD SiC has a coefficient of thermal expansion close tothat of RB SiC and, when applied only to the reflector surface 111 ofthe mirror substrate 110, can allow the optical form to be held over theoperational temperature range. Compared to amorphous, elemental siliconas a cladding for optical surfaces, which is the industry standard, CVDSiC provides better surface quality and can be polished in less time.The combination of the RB SiC mirror substrate 110 and the CVD SiCcladding 113 for the optical surface 112 can provide a relativelylow-cost, high-quality mirror.

In some embodiments, the SiC optical surface 112 can be subjected togrinding (e.g., loose abrasive grinding) and/or polishing (rough and/orfine polishing) operations to achieve a desired surface quality. Anoptical coating (e.g., a high-reflectivity, thin film coating) can thenbe applied or coated on the optical surface 112 as typical and known inthe art for precision optics.

A method 303 for preparing an optical surface of an aerospace mirror(e.g., to achieve final dimensions, form, and/or surface finish) isoutlined in FIG. 5. In general, the mirror 100 (e.g., RB SiC substrate110 with CVD SiC cladding 113 as disclosed herein) has an opticalsurface 112 and one or more mounting interface surfaces 116 operable tomount the mirror 100 to an external support structure (e.g., a meteringstructure of a satellite) in a final installation by coupling with afinal installation mounting structure (e.g., flight mounts or mountingstruts). The method 303 can comprise assembling at step 330 theaerospace mirror 100 to a test support base using a test mountingstructure that is the same as, or equivalent to, or that otherwisecorresponds to the final installation mounting structure, measuring theoptical surface 112 (e.g., for form error, surface roughness, surfacequality, etc.) at step 331, and machining the optical surface 112 (e.g.,milling, grinding, and/or polishing) at step 332. Measuring 331 theoptical surface 112 and machining 332 the optical surface 112 can berepeated until the optical surface 112 is within a given tolerance.

Aspects of an aerospace mirror manufacturing assembly 404 that can beused to prepare an optical surface 412 of an aerospace mirror 400 areillustrated in FIGS. 6-9. In particular, FIGS. 6-8 show schematicrepresentations of front, side, and rear views, respectively, of themirror manufacturing assembly 404, and FIG. 9 shows a detail view of themirror 400 coupled to a mounting structure.

In addition to the aerospace mirror 400, the assembly 404 can include atest support base 440 and a test mounting structure 441 (e.g., mountingstruts). The mirror 400 can be mounted to the test support base 440 viathe test mounting structure 441 prior to final polish of the opticalsurface 412. The test mounting structure 441 is the same as, orequivalent to, or otherwise corresponds to the final installationmounting structure. In other words, the mirror 400 can be mounted on atest mounting structure 441 that mimics or is similar to the finalmounting configuration so that the mirror 400 experiences the same orsimilar type and/or amount of deformation during the final stages ofmanufacture that it will experience in its final mounting configurationfor use. Typically, mounting a mirror in its final (i.e., flight)mounting configuration (e.g., on a satellite) will induce significantwavefront error due to deformation of the mirror's optical surface,which may be due to mounting hardware misalignment and/or bonding errorsresulting from bonding mounting hardware to the mirror. Mounting themirror in a set-up that is the same or similar to the final mountingconfiguration causes this deformation to occur during manufacturing(e.g., prior to final polishing). As a result, the final materialremoval processes (e.g., final polishing) can account for the assembleddeformation. In other words, the mirror can be in its final as-assembledconfiguration during the final stages of manufacture so that anydeformation introduced in this configuration can be removed bysubsequent material removal processes. Accordingly, the final mirrorproduced is within tolerance when actually mounted in its final (i.e.,flight) configuration, such as on a satellite. This approach cantherefore introduce fewer errors in the overall assembly error stack-upand prevent mounting and bonding error from being evident in the finaloptical surface.

The test mounting structure 441 can have any suitable configuration. Insome embodiments, the test mounting structure 441 can comprise one ormore struts 442 a-f. Examples of such struts are disclosed in UnitedStates Patent Application Publication No. 2017/0055731 referred toabove. In the test mounting structure 441 configuration, the number ofstruts 442, the angle/orientation of the struts 442 relative to themirror 400 (e.g., to ribs 415), the length of the struts 442, etc. canbe the same or configured to produce an equivalent effect on the mirror400 as the final mounting structure. In some embodiments, one or morestruts 442 can have an adjustable length along an axis 445 (FIG. 9) tofacilitate adjusting a position of the mirror 400. In some embodiments,one or more struts 442 can have a negative coefficient of thermalexpansion (CTE). Examples of such struts are disclosed in U.S. patentapplication Ser. No. 15/828,223 filed Nov. 30, 2017 and titled“Multi-Material Mirror System,” which is incorporated herein byreference in its entirety.

In a particular embodiment, the struts 442 a-f can be coupled to supportribs 415 of the mirror 400, as shown in FIG. 7. As illustrated, thestruts 442 a-f can be aligned with the ribs 415 to which they areattached to efficiently transfer loads between the mirror 400 and thetest support base 440. In this case, six struts 442 a-f are arranged inthree sets or pairs (e.g., bi-pods), which are aligned with the ribs 415or rib planes 443 a-c oriented in three directions. In addition to thestruts 442 a-f, the test mounting structure 441 can include attachmentfittings 444 (e.g., a clevis, see FIG. 9). The attachment fitting 444can be bonded (e.g., with structural adhesive) or otherwise attacheddirectly to the ribs 415 to which the struts 442 a-f are aligned. Thus,the mirror 400 can have a mounting interface surface 416 (e.g., a bondpad) coupled to the test mounting structure 441.

With the struts 442 a-f in-line with the ribs 415 or rib planes 443 a-c,out-of-plane loads can be minimized or prevented from being impartedonto the ribs 415. Such a configuration can achieve an extremelyefficient load transfer path from the strut mounts into the mirror 400and can enable the mirror's optical form to be minimally affected byoperational temperature changes, particularly when low CTE materials areutilized in the mounting structure (e.g., Invar® attachment fittings444). This strut configuration can be identical or kinematicallyequivalent to a final mounting configuration for the mirror wheninstalled, for example, on a satellite. The stiff RB SiC material of themirror 400 and its lightweight design can facilitate the use oflightweight kinematic mounts, such as the struts 442 a-f, which canminimize thermal sensitivity and self-weight deflection of the mirror400.

In one aspect, the hardware used to mount the mirror 400 to the testmounting structure 441 can be the same hardware used to mount the mirror400 in its final mounting configuration when installed. For example, thestruts 442 a-f used to mount the mirror 400 in the assembly 404 can alsobe used to mount the mirror 400 in its final flight mount configuration.In addition, the attachment fittings 444, which may be bonded to themirror 400, can remain permanently attached to the mirror 400 even ifdifferent struts may be used in the final flight mount hardware.

With the mirror 400 mounted as in, or equivalent to, the final flightmount configuration in the assembly 404, the optical surface 412 can bemeasured by optical testing of the optical surface 412 (e.g., opticalmetrology to ascertain form error). Any suitable optical test ormetrology technique can be utilized. In some embodiments, a rotationalshearing optical metrology technique can be employed. Rotationalshearing can provide high accuracy in determining the mirror's surfacefigure while reducing the influence of gravity in the flight mountingconfiguration, while also allowing use of a low-cost horizontal opticaltest. Thus, the optical metrology for the mirror 400 can be done usingrotational shearing techniques to remove the influence of gravity whilethe mirror 400 is mounted in the assembly 404.

The optical surface 412 can be fine or finish machined while the mirror400 is in the assembly 404 or, in other words, after mounting hardwarehas been attached to the mirror 400 (e.g., attachment fittings 444 havebeen bonded to the bond pads 416 and coupled to structural supports,such as the struts 442 a-f) and the mirror 400 has been mounted to thetest support base 440. The optical surface 412 can be finish machinedutilizing any suitable process or technique, such as polishing,grinding, lapping, etc. In one aspect, the assembled mirror 400 can bepolished using rotational shearing techniques to remove the influence ofgravity while mounted on the structural supports. This can account forany mechanical influence of the structural supports on the optical form.In other words, any mechanical influence of the mounting structure 441on the optical form can be accounted for in the manufacturing processand removed during final machining processes (e.g., polishing). The useof flight or flight-like mounts during manufacturing and optical testingcan therefore remove at least one factor from the potential optical formerror sources.

In one aspect, the CVD SiC cladding material can facilitate polishingthe optical surface. For example, the CVD SiC cladding that forms theoptical surface 412 is easier to polish than the RB SiC material thatforms the mirror substrate 110 of FIGS. 1A, 1B, 2, and 3. The CVD SiCcladding 113 can enable the optical surface 412 to be polished to betterthan 5 angstroms of RMS surface roughness, with excellent optical formand control of the optical surface 412.

Once the optical surface 412 is in an acceptable condition, the opticalsurface 412 can then be coated with a suitable optical coating (e.g., ahigh-reflectivity thin film coating) as known in the art.

Reference was made to the examples illustrated in the drawings andspecific language was used herein to describe the same. It willnevertheless be understood that no limitation of the scope of thetechnology is thereby intended. Alterations and further modifications ofthe features illustrated herein and additional applications of theexamples as illustrated herein are to be considered within the scope ofthe description.

Although the disclosure may not expressly disclose that some embodimentsor features described herein may be combined with other embodiments orfeatures described herein, this disclosure should be read to describeany such combinations that would be practicable by one of ordinary skillin the art. The user of “or” in this disclosure should be understood tomean non-exclusive or, i.e., “and/or,” unless otherwise indicatedherein.

Furthermore, the described features, structures, or characteristics maybe combined in any suitable manner in one or more examples. In thepreceding description, numerous specific details were provided, such asexamples of various configurations to provide a thorough understandingof examples of the described technology. It will be recognized, however,that the technology may be practiced without one or more of the specificdetails, or with other methods, components, devices, etc. In otherinstances, well-known structures or operations are not shown ordescribed in detail to avoid obscuring aspects of the technology.

Although the subject matter has been described in language specific tostructural features and/or operations, it is to be understood that thesubject matter defined in the appended claims is not necessarily limitedto the specific features and operations described above. Rather, thespecific features and acts described above are disclosed as exampleforms of implementing the claims. Numerous modifications and alternativearrangements may be devised without departing from the spirit and scopeof the described technology.

What is claimed is:
 1. A method for preparing an optical surface of anaerospace mirror, comprising: obtaining an aerospace mirror having anoptical surface on a front side and a mounting interface surface on aback side, the mounting interface surface facilitating mounting of theaerospace mirror to an external support structure in a finalinstallation by coupling with a final installation mounting structure;assembling the aerospace mirror to a test support base by coupling atest mounting structure to the mounting interface surface, wherein thetest mounting structure corresponds to the final installation mountingstructure; measuring the optical surface; and machining the opticalsurface.
 2. The method of claim 1, wherein measuring the optical surfaceand machining the optical surface are repeated until the optical surfaceis within a tolerance.
 3. The method of claim 1, wherein measuring theoptical surface comprises optical testing of the optical surface.
 4. Themethod of claim 1, wherein machining the optical surface comprisespolishing, grinding, lapping, or a combination thereof.
 5. The method ofclaim 1, wherein the test mounting structure comprises a strut with anadjustable length.
 6. An aerospace mirror, comprising: a reaction bonded(RB) silicon carbide (SiC) mirror substrate; and a SiC cladding on theRB SiC mirror substrate forming an optical surface on a front side ofthe aerospace mirror.
 7. The mirror of claim 6, wherein SiC claddingcomprises a chemical vapor deposition (CVD) SiC.
 8. The mirror of claim6, wherein the SiC mirror substrate comprises a plurality of ribs on aback side of the aerospace mirror opposite the front side.
 9. The mirrorof claim 6, further comprising an optical coating on the opticalsurface.
 10. An aerospace mirror manufacturing assembly, comprising: atest support base; a test mounting structure; and an aerospace mirrormounted to the test support base via the test mounting structure, theaerospace mirror having an optical surface on a front side and amounting interface surface on a back side coupled to the test mountingstructure, wherein the mounting interface surface is operable to mountthe aerospace mirror to an external support structure in a finalinstallation by coupling with a final installation mounting structure,and the test mounting structure corresponds to the final installationmounting structure.
 11. The manufacturing assembly of claim 10, whereinthe test mounting structure comprises a strut with an adjustable length.12. The manufacturing assembly of claim 11, wherein the strut has anegative coefficient of thermal expansion (CTE).
 13. The manufacturingassembly of claim 11, wherein the strut comprises six struts.